PASADENA, Calif.--Planetary scientists at the California Institute of Technology and Yale University on Tuesday night discovered a new planetoid in the outer fringes of the solar system.

The planetoid, currently known only as 2004 DW, could be even larger than Quaoar--the current record holder in the area known as the Kuiper Belt--and is some 4.4 billion miles from Earth.

According to the discoverers, Caltech associate professor of planetary astronomy Mike Brown and his colleagues Chad Trujillo (now at the Gemini North observatory in Hawaii), and David Rabinowitz of Yale University, the planetoid was found as part of the same search program that discovered Quaoar in late 2002. The astronomers use the 48-inch Samuel Oschin Telescope at Palomar Observatory and the recently installed QUEST CCD camera built by a consortium including Yale and the University of Indiana, to systematically study different regions of the sky each night.

Unlike Quaoar, the new planetoid hasn't yet been pinpointed on old photographic plates or other images. Because its orbit is therefore not well understood yet, it cannot be given an official name.

"So far we only have a one-day orbit," said Brown, explaining that the data covers only a tiny fraction of the orbit the object follows in its more than 300-year trip around the sun. "From that we know only how far away it is and how its orbit is tilted relative to the planets."

The tilt that Brown has measured is an astonishingly large 20 degrees, larger even than that of Pluto, which has an orbital inclination of 17 degrees and is an anomaly among the otherwise planar planets.

The size of 2004 DW is not yet certain; Brown estimates a size of about 1,400 kilometers, based on a comparison of the planetoid's luminosity with that of Quaoar. Because the distance of the object can already be calculated, its luminosity should be a good indicator of its size relative to Quaoar, provided the two objects have the same albedo, or reflectivity.

Quaoar is known to have an albedo of about 10 percent, which is slightly higher than the reflectivity of our own moon. Thus, if the new object is similar, the 1,400-kilometer estimate should hold. If its albedo is lower, then it could actually be somewhat larger; or if higher, smaller.

According to Brown, scientists know little about the albedos of objects this large this far away, so the true size is quite uncertain. Researchers could best make size measurements with the Hubble Space Telescope or the newer Spitzer Space Telescope. The continued discovery of massive planetoids on the outer fringe of the solar system is further evidence that objects even farther and even larger are lurking out there. "It's now only a matter of time before something is going to be discovered out there that will change our entire view of the outer solar system," Brown says.

The team is working hard to uncover new information about the planetoid, which they will release as it becomes available, Brown adds. Other telescopes will also be used to better characterize the planetoid's features.

2004 DW is a newly discovered Kuiper belt object, found in images taken Feb 17 2004 (nearly 74 years to the day after Pluto was discovered) by Mike Brown (Caltech), Chad Trujillo (Gemini Observatory) and David Rabinowitz (Yale). We are the same people who found Quaoar in the summer of 2002. 2004 DW is one of the largest Kuiper Belt objects currently known. Initial indications are that it may be over half the diameter of Pluto, larger than Quaoar, and 2.4 billion kilometers (1.5 billion miles) further away than Pluto.

How big is 2004 DW?

So far, we are not sure. The size has not been accurately measured like the size of Quaoar has. However, we know the object's distance very roughly as well as its brightness. Using this and our best guess at the object's albedo (how light or dark the surface is) of 9%, the object is probably about 1600 km in diameter, larger than the 1250 km Quaoar. If subsequent measurements verify this size estimate, this would make 2004 DW the largest minor planet known, and larger than Pluto's moon Charon, which is about 1300 km in diameter. This still doesn't beat Pluto, which is about 2300 km in diameter.

Is 2004 DW a Planet?

We don't think so. It's probably slightly larger than half the size of Pluto and further away, but there are other big objects out there, too. Quaoar (1250 km in diameter), Varuna (900 km in diameter) and 2002 AW197 (also 900 km in diameter) are three of the biggest Kuiper Belt Objects, and they are not considered "planets" either. There are about 800 known Kuiper Belt Objects, most of which are only about 100 km in diameter, and all of which were discovered since 1992 by different scientists who have been looking for them. It's similar to the asteroid belt, but beyond Neptune and contains maybe 100 times more material. You can read more about the Kuiper Belt at the Kuiper Belt Homepage. You can also see the article I wrote for the Sep-Oct 2003 issue of American Scientist.

Note that the Kuiper Belt is also called the Edgeworth-Kuiper Belt and the trans-Neptunian Belt. We think we should be able to find 5 - 10 more of these really big Kuiper Belt Objects over the next couple of years (we are calling them "planetoids"), including perhaps a couple "super-Plutos".

Just what is a planet, anyway?

In truth, scientists have been discussing this for a few years now, and currently no consensus has been reached. The real problem is that these "planetoids" like 2004 DW and Quaoar really blur the line between planet and asteroid. The are generally too large to be called asteroids because they approach the size of Pluto, which has been called a planet for more than the last 70 years. No one really wants to demote Pluto as a planet because of historical precedents, but what do we do if we find something slightly larger than Pluto? Do we create a new planet? What if we find 5 things slightly larger than Pluto? Are they all planets? To date these are unanswered questions.

How was 2004 DW found?

We have been conduction an ongoing survey of the outer solar system using the Palomar QUEST camera and the Samuel Oschin Telescope at Palomar Mountain in Southern California. This survey has been operating since the fall of 2001, with the switch to the QUEST camera happening in the summer of 2003. To date we have found around 35 bright Kuiper belt objects.

How far away is 2004 DW?

2004 DW is at about 45 AU away from us, more distant than Pluto and Neptune, which are both at about 30 AU. 1 AU is an "Astronomical Unit" and is equal to the distance between the Earth and the Sun, about 150 million kilometers. So 2004 DW is about 7 billion kilometers from us. At walking speed (if you could walk in space), it would take you about 150,000 years to get there. Going at the speed the Space Shuttle orbits the earth, it would take about 30 years to get there. It takes light 6 hours to get there from the sun.

What is 2004 DW's orbit like?

Since its discovery (February 2004), we have been trying to locate serendipitous images of the object in old datasets. We have so far traced its orbit back to 2002. All indications are that it is a "Plutino" --- that is a Kuiper belt object in an orbit very similar to Pluto's: Pluto goes around the sun twice for every 3 times that Neptune goes around the sun. The same is true of 2004 DW. Also, the longitude of perihelion is close to Pluto as well. Pluto's twin? Not quite, but not too far off!

How bright is 2004 DW?

The brightness of 2004 DW is still only known roughly from the discovery images. We estimate that it is about 18.5 magnitudes in R, about the same brightness as Quaoar. Since it is further away from us than Quaoar, it is likely to be larger than Quaoar.

What is 2004 DW made of?

We don't know. We suspect that most Kuiper Belt Objects are made of equal portions of rock and ices. There are many chemicals which are normally liquids or gas on Earth that would be different types of ice on 2004 DW, including water, methane ice (natural gas ice), methanol ice (alcohol ice), carbon dioxide ice (dry ice), carbon monoxide ice and others.

Technology is the reason. Clyde Tombaugh discovered Pluto in 1930 using photographic plates, which let you look at a very wide piece of the sky, but they are not nearly as sensitive as the CCD's that we use now. (A CCD is what you will find inside most digital cameras.) The new, large objects listed above tend to be just faint enough that they would be out of range of all the older surveys for moving objects done after Tombaugh's. Today, CCD's are getting large enough and computers are getting fast enough that it is significantly easier to find these types of planetoids than it was even 5 years ago. We use a 172 Megapixel camera mounted on a robotic telescope to find these things. Even about 5 years ago, such cameras were not available, and the computing power to analyze these cameras was not quite there either.

Are there more big Kuiper Belt Objects like 2004 DW that we haven't seen?

It is very likely that there are more big Kuiper Belt Objects (we are calling them "planetoids", but you can call them whatever you want). We have looked at only 15% of the sky before finding 2004 DW. We have also found Quaoar which is very large. We think there could be around 5 more planetoids out there that are somewhere between the size of Charon and Pluto. It is also likely that there are a few "super-Plutos" are out there waiting to be discovered.

Why is it called 2004 DW?

This is due to rules of the Minor Planet Center and the International Astronomical Union. (There's always rules about everything, huh?) When any object orbiting the sun (asteroid, planetoid, ball of twine) is first discovered, it is given a "code name" like 2004 DW. This name is the year of discovery, followed by the half-month (almost a fortnight) of discovery (A = the first half-month of the year, B = the second half-month of the year, etc.) and then the next letter is assigned sequentially. The first object gets an 'A', the second object gets a 'B', etc. So 2004 DW was the 23rd object found in the fourth half-month of 2004. Pretty arcane, eh?

Once the orbit is better known (this will be probably within the week for 2004 DW), then the object is given a number, currently a number smaller than 100,000. The numbers aren't particularly meaningful, although Ceres (the first asteroid ever discovered) is numbered 1 and Quaoar (one of the other really large KBOs we found) is numbered 50,000. After the object is numbered, then the discoverers (that's us) have one decade to propose a name to the Internation Astronomical Union. There are even more rules about the name of the object. 2004 DW, for instance, must be named after an underworld deity because it is in a Pluto-like orbit.